Spot the mistake in the following sentence: Ian Bostridge has, when I phone him, just finished the first day of six weeks' rehearsal for English National Opera's production of Jana cek's song cycle The Diary Of One Who Vanished. OK, it's not really a mistake. But six weeks' rehearsal? For a song cycle? That's more rehearsal time than many full-scale operas can squeeze out of today's habitually crammed production budgets. This, however, is no ordinary song cycle gig. Not only will the Janacek will be fully staged, directed by Deborah Warner and sung in a new translation by Seamus Heaney, but Bostridge is no ordinary tenor, filling in the time between operas with the odd song recital. On the contrary, he has made his name, and a considerable name at that, primarily as a singer of songs - a much rarer state of affairs, in the world of professional singing, than you might imagine.~

He is full of enthusiasm for the project, as befits a man who lends his name only to those theatrical enterprises which particularly interest him, and describes Heaney's text as "beautiful". As always with Janacek, however, there will almost certainly be a bit of a kerfuffle fitting the words to the music: "I think we may end up with Seamus publishing a `poetic' version and then there might be a singing version, which is very slightly different, because in some places the music is so set to Czech patterns of speech that there's no way of finding an English equivalent. But we're also changing the music to fit the words, up to a point, because it's important that it sounds like English and not like gibberish." This is one of Bostridge's favourite topics; the sort of aww-full-ee po-li-te, stretched-out, phoney English which is often associated with art song in the vernacular and is notably absent from his own recordings of Britten and Vaughan Williams.

But given the intricacy of those Czech speech rhythms, isn't there a case to be made for doing Janacek in the original? "Well, I've done The Bartered Bride, by Smetana," says Bostridge, "and although I enjoyed it, you just learn sounds - and it's not very satisfactory if you're interested in expressing things through words. I'm sure I could get competent enough to make sounds that would sound like Czech, but the right things wouldn't be going through my head." The Diary of One Who Vanished is about a young man who abandons his home for the love of a gypsy girl. Bostridge sings The Young Man, and the Gypsy will be sung by the young British mezzo Ruby Philogene; the accompanist will be Julius Drake, Bostridge's regular pianistic partner on recital discs, including the recording of Schumann's Dichterliebe which won a Gramophone award last year. Bostridge came late to singing, having worked as a post-doctoral fellow in history at Corpus Christi College, Oxford - his book Witchcraft and Its Transformations was published by Oxford University Press in 1997 - before embarking on a full-time career as a singer in 1995.

It's unusual to make a living - let alone a career - out of singing Lieder, so why did he go down that road to begin with, rather than take the more conventional opera route? "It was Lieder that kept me interested in singing through my teenage years and into my twenties when I was an academic," he explains. "And I suppose I was lucky in that I had a job that was flexible enough for me to dip my toes into the water; and so I was able to establish a reputation as a Lieder singer before I got sucked into the whole operatic treadmill. Because the big problem is that opera is the bread and butter of the singer now, and opera as a machine isn't designed with individuals in mind - it's a big institutional thing."

The success of the Lieder side of his career has, he says, given him the clout to restrict the number of theatrical projects he take on board at any given time. "It's not that I'm not interested in theatre - it's more that I find some of the theatrical aspects of opera difficult to take. And the great thing about working with someone like Deborah (Warner) is that it's theatre without compromise. For her, it has to be real theatre - it can't be some sort of operatic hybrid." Hang on a second. Don't most singers find Lieder an over-disciplined, even somewhat stuffy form? Don't most people claim to be able to express themselves better on the operatic stage? "I suppose so - I mean, some people say they're happier doing opera because they can pretend to be somebody else, but my only way to cope with acting has been to see that acting isn't really about being somebody else, but about using yourself in different ways, finding aspects of yourself that fit certain dramatic situations.

I suppose there are two sorts of acting; and that's where I come in." As for the hothouse blacktie atmosphere of a song recital, he says it can, in a weird sort of way, be liberating. "I think that's something you can play with, which is quite nice; the fact that it's so formal and that you're dressed up in such formal clothes gives you a line which you can rebel against. Some people think that what I'm doing is outrageous and offensive because I move about more than most Lieder singers do - but I like that you can actually jolt people out of the complacency of the form. I don't sing in a detached way, and some people don't like that either, but that's the only way that I can do it, so . . . " Check out "the only way that he can do it" on his superb Schubert disc for EMI - and be prepared to be dazzled. But would he agree that the greatest joy of Lieder is its inwardness - that, as a form, it has a limited potential for theatricality? He wouldn't, really. "Yeah," he says doubtfully, "but I think, as a singer, you have to grab them, because it's very easy to sit back and not be involved.

You have to make people listen to every single thing you sing, and not just drift in and out of this nice background music. For me that's the whole point of song recitals; and that's where the structure of recitals is very important, and pacing, and the way you move from one song to the next - so that it isn't just a polite series of isolated events with a cough and a splutter in between." Not much danger of that in the Janacek cycle, anyhow, with Deborah Warner directing operations and the piece being designed and lit by Jean Kalman. Bostridge worked with Warner on Benjamin Britten's chamber opera The Turn of the Screw, and says her influence on his stage work has been incalculable. "She gave me the confidence to feel that what I did in a recital was really the same as what I should be doing in opera, and to break down the barriers and perform as an actor - which was a real transformation for me, because I'd always thought of myself as somebody wooden, and not the sort of person who could act. "In one sense, if you're put in the right hands anybody can act. Rehearsing this Janacek piece, we've been doing lots of acting exercises. And you begin to see the way in which you can approach something that might be called `acting'. Through telling stories - or doing all sorts of things that are a lot less intimidating than being told to get up on a stage and be angry."

The Diary of One Who Vanished will be performed at the Gaiety Theatre, from October 14th-16th